Solid state lasers that are optically pumped by diode lasers have gained an increased attention during the last years. Solid state lasers are often favored in comparison to diode lasers, because a much better beam quality is obtained from solid state lasers. In order for the optical pumping of a solid state laser to be efficient, it is important to match the beam size and beam shape of the pump beam to the size and shape of the transverse area of the lasing mode in the solid state laser. If the pump beam is larger than the lasing mode, there is a loss of energy because not all energy supplied by the optical pumping can be extracted into the lasing mode. For three-level or quasi three-level lasers, there is also a problem if the pump beam is smaller than the lasing mode. In this latter case, no gain is provided to the lasing mode outside the volume occupied by the pump beam, but instead absorption losses occur due to reabsorption of energy from the lasing mode.
For the above reasons, the size and shape of the pump beam in a diode pumped solid state laser have generally been adjusted in order to match the lasing mode of the solid state laser to the largest possible extent. To this end, various types of focusing arrangements have been employed.
One example of a beam shaper for shaping a beam from a diode laser for the purpose of pumping a solid state laser consists of a thick cylindrical lens that is arranged with its principal axes rotated 45 degrees with respect to the principal axes of the astigmatic diode beam. An arrangement of this type is sometimes called a “beam twister”, and makes an initially astigmatic beam rotationally symmetric by beam twisting. A theoretical background to beam twisting by means of a thick cylindrical lens is given by Laabs et al. in the article “Twisting of three-dimensional Hermite-Gaussian beams”, Journal of Modern Optics, 1999, vol. 46, no. 4, pp 709–719.
Diode lasers in general have very astigmatic emissions, with a highly elliptical beam (large difference in M2-values). The dimension having the highest degree of divergence in the emission from a diode laser is referred to as the fast axis, and the dimension having the lowest degree of divergence is referred to as the slow axis. In order for the beam twister to function properly according to the above-mentioned theory for standard thick lenses, the fast and the slow axis must somehow be made to have similar Rayleigh lengths inside the beam twister. This is generally obtained by means of two intermediate cylindrical lenses arranged between the diode laser and the beam twister. The first of the two intermediate lenses converges the emission in the fast axis, and the second converges the emission in the slow axis. In this way, both the fast and the slow axis can be focused simultaneously inside the beam twister such that a rotationally symmetrical beam is ideally obtained behind the twister.
The above-mentioned arrangement for beam twisting has proven to be adequately effective in order to provide a beam for pumping of solid state lasers. However, the cylindrical lenses used makes the arrangement very bulky. Moreover, cumbersome alignment of each individual lens is usually required.
Therefore, there is a need for improved arrangements for shaping the output beam from a diode laser. In particular, there is a need for more compact arrangements for beam twisting an astigmatic beam into an ideally rotationally symmetric beam. More compact arrangements are particularly attractive for compact laser sources, where lower cost, automation of manufacturing, and simpler alignment of optical elements are some of the advantages. Furthermore, diode-pumped solid-state lasers are often incorporated as sub-assemblies in other instruments, further increasing the requirements on compactness. In addition, the beam of high rotational symmetry obtained by the present invention may be used directly for various purposes.